Cavity resonator arrangement



Aug. 7, 1951 s. NERGAARD CAVITY RESONATOR ARRANGEMENT Filed Aug, 6, 1947 INVENTOR I LzozvJ/I/mamm BY Patented Aug. 7, 1,951

CAVITY RESQNATOR ARRANGEMENT Leon S. Nergaard, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 6, 1947, Serial No. 766,649

6 Claims.

,flhis invention relates to cavity resonator arrangements suited for production of high power at ultra-high frequencies and particularly concerns oscillators, power amplifiers and the like which utilize electronic tubes and external cavity, resonators.

I It is an object of the invention substantially to extend the range of frequencies for which cavity resonators may be used with triodes and other "tubes not of the magnetron type.

With the usual cavity. resonators, it is increasingly difficult at higher and higher frequencies to obtain satisfactory performance because the resonator becomes so small that there is insuflicient space therein to use coupling and tuning. elements which are effective; moreover, thesinall dimensions of the cavity definitely limit the power which can be produced.

.i' In accordance with the present invention, a cavity resonator employed in an oscillator, amplifier or other high-frequency circuit is of length corresponding with three or greater odd number of, half wavelengths at the desired operating frequency, so to obtain sufficient space within the cavity for effective coupling and tuning elements; oscillations of frequency corresponding with the fundamental mode of the cavity are suppressed by bailles so located, as at or near volta e nodes for the desired harmonic mode, that they, are equivalent to shunt elements having very low. impedances at the fundamental frequency but of very much higher impedance at thedesired operating frequency.

More particularly, and for usein an oscillator system, the resonant cavity structure comprises two cavities disposed side by side and provided with openings for insertion of electronic tubes having electrodes respectively connecting with corresponding walls of the cavities: the openings are so located that the tubes are at voltage antinodes of like instantaneous polarity for operation of the tubes in parallel relation. In one or both of the cavities, a tuning vane, or equivalent tuning element, is disposed at or near a third voltage anti-node.

" Further in accordance with the invention, the cavities may be extended in direction normal to their aforesaid length to accommodate one or more additional pairs of tubes operating in parallel with the first-mentioned tubes so to obtain correspondingly increased power output from the single oscillator system.

:,The invention further resides in features of construction, combination and arrangement hereinafter described and claimed.

For more detailed understanding of the invention, reference is made to the accompanying drawings in which:

Fig. 1 is a plan view of a cavity-resonator for use with two pairs of electronic tubes;

Fig. 2 is a front sectional view taken on line 2 z of Fig. 1; r

Fig. 3 is an end sectional view taken on line 3-3 of Fig. 1; and I Figs. 4 and 5 are explanatory figures referred to in discussion of the invention.

Referring to Figs. 1 to 3 as exemplary of a system embodying the invention, the resonant cavity structure [0 is a rectangular metal box having parallel top and bottom walls I l,,-l2 and an intermediate wall I 3 parallel thereto and dividing the box into an upper cavity resonator land a lower cavity resonator I5. The length of these cavities as measured between the end walls I6 I6 is three or greater odd number of half wavelengths at the desired operating frequency. In the particular arrangement shown, the length is three half wavelengths: for operation at frequencies in the range of from about 1000 megacycles to about 1200 megacycles this dimension is about 7.5 inches.

The sides of the cavities are closed by the walls I1, H, the width of the cavity in the particular arrangement shown in Figs. 1 to 3 being sufficiently great to accommodate two elongated electrode electron tubes disposed with theirmajor axes in alignment and parallel to the ends I6.

of the cavity structure. Each of the openings I8 in the upper wall II of the resonator structure is in alignment with one of the holes I9 of.

U. S. Patent No. 2,481,026 of Russell R. Law, et a1. The anode of each tube is effectively connected;

as by a bypass condenser (not shown) to the outer upper wall II; the grid of each tube is connected, as by resilient contact fingers (not shown), to the intermediate wall l3; and the cathode of each tube is connected, as through a bypass condenser, to the outer bottom wall I2 of the resonator structure. The grid electrode of the tube may be defined as the control element which controls the flow of electrons from cathode to anode. The anodes of the tubes may 7 be connected through the usual radio-frequency filter chokes to the positive terminal of a source of direct current whose negative terminal is con.-;

nected to the cathodes either directly or through cathode resistances. A direct-current path to the grids of the tubes may be afforded by resistor, not shown, connected between the resonator casing and the negative terminal of the anode source of direct current.

Thus the upper cavity l4 defined by. the upper wall sand thecommon wall l3 forms a;grid

anode cavity common to all of the tubes, and the lower cavity I5 forms a grid-cathode cavity come mon to all of the tubes. As above stated, the length of each of the cavities between the end walls I6 may be three half wavelengths at the desired operating frequency andinsuch case-the openings 18 are so located that the electrodes of each tube are one-quarter wavelength from the corresponding end l6 of the cavity and :areone wavelength from the electrodes of the other tube of the pair. This spatial relation is more clearly shown in Figs, the electrodes of tube 21A being, as appears from curveH, at a voltageanti-node X t me is "one quarter wavelength from the lefthand end wall lfi whereas the electrodes of the other t u be -2IB ofythe pair are at a voltage antinode Y located one-quarter wavelength from the 'righthand end wall [6. The corresponding electrodes of the two tubes are therefore of the same instantaneous polarity. Accordingly, the phase relations are such that the tubes function in parallel andthe output is therefore substantially greater than that obtainable from a cavity reso'natorexcitedfrom a single' tube. The tubes 21C and; 2 ID are similarlydisposed and also operate in parallel relation-with respect to each other and ere 2 -1 "*Tuni-ng-wanes 25 and 26 are disposed at or near a voltageanti-nqde Z of curve H intermediate the-anti-nodes X, Y at which the tubes are disposed. 'lhe' dimensions of the cavity are such that the--tuning vanes 25, 26 respectively associated -with-wthecavities I 4- and [5- may be of physical size affordinga satisfactorilylarge tuning ranger; forexamp le, in the particular arrangementshownthe tuning range was from about 1000 megacy'cles toabout 1200 megacycles.

Moreover, and also in contrast with the conventional cavity resonator arrangements, there is ample-space; for provision ofeffective feedback coupling-between theftwo' oscillator cavities. In the-particular arrangement shown. thefeedback couplingre'quired for-operation of the: arrangements as an oscillatoris provided by the rod 22 which 'threadedly engages the lowerouter wall- I 2- of resonator. I 0 and extends through the open- 11:18:23: in -t'he intermediate Wall l3 of the two cavities; Thedesiredmagnitude of the feedback coupling .--may be :efiected by adjustment of the knob 24 attached to rodaz 2 and accessible externally. ofathe: cavityresonator I 0. This feedback coupling/means, because of the enhanced length of the "cavities, is physically remote from the tubes.

-Theenhanced dimensions of theresonator: also facilitatelitscouplingtothe load; In the particularjarrangement shown, power isabstracted from the.-cavity byraloop formed at least in part bytheinner; conductor" 3 of I a concentric transmission line ;29a whose -outer conductor 30 is conductively connected to the cavity resonator structure, specifically.-the.; upperwall H thereof. With the resonatorestructure shown and using four tubes of .-thee.typ.e disclosed in: theaforesaid application of -:La w--.et-al;, theescillator output of about 1 megawatt, that is 1000 kilowatts, may be expeeted.

Generation of oscillations at the fundamental mode of the cavity, represented by curve F of Fig. 4, is effectively prevented by the short metallic baiile elements 21 which extend inwardly from the side walls I! of the resonatorstructure l0 and by the intermediate bafiies 21A in alignment therewith. ,fIThe seriesofaligned baffles nearer the tube ZIA are disposed at one zvo'ltage node A of the harmonic curve H of Fig. 4, whereas the series of bafiles nearer the tube ZIB are disposed at a second voltage node B. These baflies appear to oscillations of the fundamental mode F of the cavity as. shunt impedances of very low magnitude and so .eflectively suppress the undesired oscillations. ,Thewidth of the opening between the adjacent ends of each pair of aligned baflles of eachseries is-sogreat the baflies have inapprociable eiTect upon oscillations of the desired harmonic frequency of operation.

For a better understanding of the effect of' the baffles, an equivalent circuit arrangement having lumped reactances is shown in Fig. .5; "the: impedances 32- correspond with the shunt impedances aflorded by the :baflies 21 and 21a of Eig's. 1 to 4. The condenser 33corresponds withthe feedback capacityafforded by the icou'pling rod 22,, andthe variable condensers 34 and 35. correspend with the variable capacity"effectiofthe tuning vanes 25 and-26. "The 'condens'ers'31 and 38 correspond with theanodeand'cathode bypass condensers, respectively.. The. other. inductances and capacities shown in Fig. 5.: correspond with the distributed inductive and capacitive effects of the resonant cavities l4-and I5. lEachrofthe cavitiesis therefore equivalent 'to a filter network for attenuating the fundamental resonanfiffequency of the cavity.

It shall be understood that'if' a'single pair: of tubes such as Z IA; 21B affords'the power' desired, the cavity resonator ill-maybe correspondingly reduced in width and theintermediate 'b'aflle's' 21A omitted; if additional power output is required; the width of the cavity resonator may becomespondingly; increased to" accommodate one-for more additional pairs of'tubes. 'In the 'latter case, of course, additional'baifies- 21A shouldilie provided to suppress" oscillations atthdfiiliddmental mode.

When" the cavity--- structure is used, a'shbdVd described as the circuitcomp'onents of" anioscil lator, ba-files are preferably-used inboth offtfid cavities. When, 'howevergit isus'edih; ai'fl' quency-mixing or irequency multiplying arrange ment, it-ma-y' bedesirable to omit thebafllesfio'm one of the cavities. Other uses ofi'the cavityresi onator will be apparent to those skillediintl'ie art.

' The length of the resonator may "be" five; sevenorgreater oddnumber or half-wavelengths" long? sired frequency of .operation. .and-Ithey. should' b ea I spaced an integral number of 'wavelengths.= .Ad-

ditionalr-bafllesshould be disposediat or .neanpoposed: between the tubes and aligned cross wise of the-tubes to ensure that -these tubes function' a a u ity and-t t e masnetid'iield attlie'cor responding end of the cavity does not break up into loops encircling the tubes individually.

Accordingly it shall be understood the invention is not limited to the particular arrangement shown, and that changes and modifications may be made within the scope of the appended claims.

What is claimed is:

1. A resonant cavity oscillator comprising electronic tubes each having respectively an anode, a cathode and a control element, and cavity resonators having outer wall structures respectively connected to the cathodes and anodes of said electronic tubes and common inner wall structure connected to the said control elements of said tubes, whereby said resonators are coupled by operation of said tube, each said resonator having an electrical length of three or greater odd number (2n+1) half wavelengths at the operating frequency and subject to oscillations at an undesired second frequency of of said operating frequency, the walls of said resonators having openings for insertion of said tubes at voltage anti-nodes of the desired frequency of operation, and metallic baflles in at least one of said resonators so dimensioned and located effectively at nodal positions for said operating frequency to appear as low shunt impedances to oscillations of said second frequency corresponding with the fundamental mode of said resonator.

2. An oscillator for generation of high-frequency oscillations comprising a cavity resonator of electrical length in one direction approximately corresponding with three or greater odd number of half wavelengths at the desired operating frequency, at least one pair of metallic baffles spaced at half wavelength intervals along said direction at nodal points at said operating frequency within said cavity structure to suppress oscillations at the fundamental mode thereof, oscillator tubes inserted in said cavity structure at voltage anti-nodes at said operating frequency of like instantaneous polarity for in-phase operation and on opposite sides of said point of baflies thereby to couple said cavities by operation of said tubes, and tuning vane structure extending into said cavity structure at a voltage anti-node of instantaneous polarity opposite to that of said first-named anti-nodes and between said pair of baflies.

3. An oscillator comprising two cavity resonators disposed side by side with a common wall portion and having a length corresponding with three or greater odd number of half wavelengths at the desired operating frequency, two pairs of electron tubes disposed within said resonators and coupling said resonators and having like elongated anode, cathode, and grid electrodes oriented transversely to said cavity length and each connected to the corresponding wall structures of said resonators with said grid electrodes being connected to said common wall portion, each of said pairs of tubes being disposed at a voltage anti-node of the same instantaneous polarity as the voltage anti-node at which the other pair is disposed and in alignment with a corresponding one of the other pair of tubes.

4. An oscillator comprising two cavity resonators disposed side by side with a common wall portion and having a length approximately three or greater odd number of half wavelengths at the desired operating frequency, two pairs of elongated electrode electron tubes to couple said resonators and having like electrodes including grid electrodes connected to corresponding wall structures of said resonators with said grid electrodes connected to said common wall portion and all said tubes disposed with their major axes normal to said length, the tubes of one pair being disposed at a voltage anti-node having the same instantaneous polarity at which the tubes of the other pair are disposed, and baiiies within at least one of said cavity resonators between said pairs of tubes at voltage nodes of oscillations of said desired frequency and appearing as low shunt impedances to any oscillations of the fundamental frequency of said cavity resonators.

5. An oscillator comprising cavity resonators disposed side by side and each with a common wall portion having a dimension along one length corresponding with three or greater odd number of half wavelengths at a desired operating frequency, baffles located in each of said cavity resonators at voltage adjacent nodes of the desired frequency, aligned transversely elongated openings in said cavity resonators at voltage antinodes of the desired operating frequency, and electronic tubes to couple said cavity resonators positioned on opposite sides of said baffles in said openings with their like electrodes including grid electrodes connected to the corresponding wall structures of said cavity resonators, said grid electrodes being connected to said common wall portion.

6. A cavity resonator structure to be excited by two pairs of electron tubes and comprising two cavity resonators disposed side by side with a common wall portion and having a length corresponding with three or greater odd number of half wavelengths at the desired operating frequency, two pairs of aligned transversely elongated openings in the wall structure of said resonators for insertion of two pairs electron tubes with elongated electrodes including grid electrodes, each of said pairs of openings being disposed at a voltage anti-node of the same instantaneous polarity as the voltage anti-node at which the other pair is disposed, said grid electrodes being connected to said common wall portion.

LEON S. NERGAARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,261,130 Applegate Nov. 4, 1941 2,284,405 McArthur May 26, 1942 2,323,201 Carter June 29, 1943 2,401,489 Lindenblad June 4, 1946 2,404,261 Whinnery July 16, 1946 2,408,409 Bowen Oct. 1, 1946 2,421,635 McArthur June 3, 1947 FOREIGN PATENTS Number Country Date 424,455 Great Britain Feb. 21, 1935 

